Re: [PATCH v3] softlockup: remove hung_task_check_count

[Mandeep Baines <msb@google.com>]

Hi Frédéric,

Yeah, that might work. I think you can even embed waiters_count inside
the lock. I think this can be
done with minimal changes and with no overhead added to the lock
functions. Maybe use bits 7-0 for waiters_count and move the readers
count up 8 bits.

To make room for writers_count I'd change read_lock from:

static inline void __raw_read_lock(raw_rwlock_t *rw)
{
        asm volatile(LOCK_PREFIX " subl $1,(%0)\n\t"
                     "jns 1f\n"
                     "call __read_lock_failed\n\t"
                     "1:\n"
                     ::LOCK_PTR_REG (rw) : "memory");
}

To:

static inline void __raw_read_lock(raw_rwlock_t *rw)
{
        asm volatile(LOCK_PREFIX " subl $0x100,(%0)\n\t"
                     "jns 1f\n"
                     "call __read_lock_failed\n\t"
                     "1:\n"
                     ::LOCK_PTR_REG (rw) : "memory");
}

I think that's all you'd need to change for read_lock. For write_lock
you'd have to subtract (RW_LOCK_BIAS - 1) instead of just RW_LOCK_BIAS
during lock and add it back for unlock. You'd also have to change the
tests a little to ignore bits 7-0.

Regards,
Mandeep

On Sat, Jan 24, 2009 at 7:52 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Fri, Jan 23, 2009 at 05:55:14PM -0800, Mandeep Singh Baines wrote:
>> Frédéric Weisbecker (fweisbec@gmail.com) wrote:
>> > 2009/1/23 Ingo Molnar <mingo@elte.hu>:
>> > >
>> > > not sure i like the whole idea of removing the max iterations check. In
>> > > theory if there's a _ton_ of tasks, we could spend a lot of time looping
>> > > there. So it always looked prudent to limit it somewhat.
>> > >
>> >
>> > Which means we can loose several of them. Would it hurt to iterate as
>> > much as possible along the task list,
>> > keeping some care about writers starvation and latency?
>> > BTW I thought about the slow work framework, but I can't retrieve
>> > it....  But this thread has already a slow priority.
>> >
>> > Would it be interesting to provide a way for rwlocks to know if there
>> > is writer waiting for the lock?
>>
>> Would be cool if that API existed. You could release the CPU and/or lock as
>> soon as either was contended for. You'd have the benefits of fine-grained
>> locking without the overhead of locking and unlocking multiple time.
>>
>> Currently, there is no bit that can tell you there is a writer waiting. You'd
>> probably need to change the write_lock() implementation at a minimum. Maybe
>> if the first writer left the RW_LOCK_BIAS bit clear and then waited for the
>> readers to leave instead of re-trying? That would actually make write_lock()
>> more efficient for the 1-writer case since you'd only need to spin doing
>> a read in the failure case instead of an atomic_dec and atomic_inc.
>>
>
>
> This is already what is done in the slow path (in x86):
>
> /* rdi: pointer to rwlock_t */
> ENTRY(__write_lock_failed)
>        CFI_STARTPROC
>        LOCK_PREFIX
>        addl $RW_LOCK_BIAS,(%rdi)
> 1:      rep
>        nop
>        cmpl $RW_LOCK_BIAS,(%rdi)
>        jne 1b
>        LOCK_PREFIX
>        subl $RW_LOCK_BIAS,(%rdi)
>        jnz  __write_lock_failed
>        ret
>        CFI_ENDPROC
> END(__write_lock_failed)
>
> It spins lurking at the lock value and only if there are no writers nor readers that
> own the lock, it restarts its atomic_sub (and then atomic_add in fail case).
>
> And if an implementation of writers_waiting_for_lock() is needed, I guess this
> is the perfect place. One atomic_add on a "waiters_count" on entry and an atomic_sub
> on it on exit.
>
> Since this is the slow_path, I guess that wouldn't really impact the performances....
>
>